In recent years, it has become apparent that large reserves of hydrocarbons are to be found in what are referred to as “unconventional” oil and gas bearing geologic layers. These unconventional layers, which include rock types such as shales, are typically not highly permeable, and therefore present formidable obstacles to production. The most common technique in use today that permits commercial production of natural gas and oil from such layers is hydraulic fracturing, also referred to as “fracing” or “fracking”. This technique can be also be applied to older wells drilled through conventional hydrocarbon-bearing layers to increase the proportion of hydrocarbons that can be extracted from them, thus prolonging well life.
The progress of a fracturing operation must be monitored carefully. Well fracturing is expensive, and the fracturing process is frequently halted once its benefits become marginal. The high pressures associated with fracturing result in new fractures that tend to follow existing faults and fractures, and can result in an uneven or unpredictable fracture zone. Fracturing fluid may also begin following an existing fault or fracture zone and then propagate beyond the intended fracture zone. Care must be taken not to interfere with existing oil or gas production wells in the area. For these and other reasons, it is important that the that the operator be able to accurately predict where the fluid injection will go.
One method of imaging fractures within geologic layers is known as “Tomographic Fracture Imaging”, or “TFI”. Very low level seismic (“microseismic”) energy emitted by the hydraulic fracturing of a geologic layer is sensed and recorded. The recorded data are used to determine the point of origin of the emitted microseismic energy and thus define the location of the fracture.
For effective monitoring of a fracturing operation, those controlling the operation need a near-real-time display of one or more attributes characteristics of microseismic data, capable of indicating the points of origin of microseismic energy in the subsurface, and the growth of a fracture network over time. Computing such an display requires an accurate estimate of the velocities at which seismic energy travels through the subsurface of the Earth in the area of interest. To enable a near-real-time result, it is very helpful to determine this velocity data before the fracturing operation begins, thus saving computational steps and time while the operation is in progress. Among other things, what is required is a method of determining the velocities of the seismic energy through the different geologic layers before the fracturing operation begins.